A photodiode, which serves as a photo receiving region, is generally known as an optical sensor for converting a light signal into an electric signal and is widely used as an optical sensor for control in a variety of photoelectric conversion devices.
Heretofore, on the surface of such a photodiode, an insulating film such as an SiO2 (SiO) film has been deposited in order to protect the surface of the photodiode and also in order to suppress incident light reflected on the surface of the photodiode as much as possible.
In recent years, as a storage capacity increases and a transfer rate increases, a wavelength of laser light for use in reproducing and recording information becomes increasingly shorter in an information recording medium such as an optical disc.
However, as a wavelength of laser light becomes shorter, an absolute photosensitivity of a photo receiving region is lowered increasingly. An absolute photosensitivity obtained at about a laser light λ=400 nm in wavelength becomes approximately half of a photosensitivity of infrared rays (λ=780 nm) used in the existing optical discs.
The reason for this is that, when it is assumed that one hundred percent of light is absorbed in the silicon of the photodiode and then one hundred percent of absorbed light is photoelectrically converted into an electric signal, a photosensitivity S is expressed by the following equation (1) and this photosensitivity is proportional to a wavelength of incident light.S=qλ/hc  (1)where q represents the amount of electric charge per electron, λ represents the wavelength of the incident light, h represents the Planck constant and c represents the light velocity.
Therefore, in order to increase a photosensitivity as much as possible in response to a laser light having a shorter wavelength, it is aimed to decrease the reflectance of laser light on the surface of the light-receiving portion to zero percent, i.e., the surface of the light-receiving portion should be made substantially equivalent to the non-reflection state such that substantially one hundred percent of light can be absorbed into the silicon.
First, a minimum reflectance R (min) at a wavelength λ and a film thickness T (min) of an antireflection film used in this case are expressed by the following equations (2) and (3) respectively, where the antireflection film is made of a single layer. Note that n1 assumes a refractive index of the antireflection film and n2 assumes a refractive index of the silicon. Also assumed that nitrogen gas (refractive index is approximately 1) is existing between a light source and the antireflection film,R(min)=(1×n2−n12)2/(1×n2+n12)  (2)T(min)=λ/(4×n1)  (3)
A film having a thickness expressed by the above-mentioned equation (3) is generally called a quarter-wave film.
A study of the above-mentioned equation (2) can reveal that the value of n12 is required to be close to the value of n2 as much as possible in order to make the reflectance R(min) as small as possible.
Having compared the reflectances R (min) of the insulating materials SiO2 and Si3N4 which are frequently used in a silicon-based process, for example, we have the following comparison results.
Since a refractive index of Si O2 is 1.47, a refractive index of Si3 N4 is 2.01 and a refractive index of silicon is 3.70 at a wavelength λ=780 nm, the results areR(min)SiO2=(3.70−1.472)2/(3.70+1.472)2=7.6%andR(min)SiO2=(3.70−2.012)2/(3.70+2.012)2=7.6%
Subsequently, let us consider the minimum reflectances in the case of a wavelength λ=400 nm in a similar fashion
Since the refractive index of SiO2 is 1.47, the refractive index of Si3N4 is 2.07 and the refractive index of silicon is 5.60 at the wavelength λ=400 nm, the results are R (min) SiO2=(5.60−1.472)2/(5.60+1.472)2=19.5% and R (min) Si3N4=(5.60−2.072)2/(5.60+2.0722)2=1.8%.
FIG. 4 shows the change in reflectance of the antireflection film having the single layer of a silicon oxide film (SiO2 film) at the wavelength of 400 nm when the film thickness of the antireflection film is changed.
It is understood in FIG. 4 that reflectance of the antireflection film having the single layer of the silicon oxide film (Si O2) is minimized to obtain the above mentioned reflectance of 19.5% when the film thickness satisfied λ(1+N)/(4×n(Si O2))=68 nm, 204 nm, 340 nm, . . . with N=0, 1, 2, . . . .
FIG. 5 shows the change in reflectance of the antireflection film having the single layer of a silicon nitride film (Si3N4 film) at the wavelength of 400 nm when the film thickness of the antireflection film is changed.
It is understood in FIG. 5 that reflectance of the antireflection film having the single layer of the silicon nitride film (Si3 N4) is minimized to obtain the above mentioned reflectance of 1.8% when the film thickness satisfiedλ(1+2N)/(4×n(Si3N4))=48 nm, 145 nm, 242 nm, . . . with N=0, 1, 2, . . .
Specifically, it is to be understood that when the antireflection film having the single layer of the Si3N4 film is formed in the selected film thickness, the reflectance can be considerably lowered at any wavelengths as compared with the case in which the antireflection film has the single layer of SiO2 film.
However, in the existing technology of silicon-based process, a method of depositing an Si3N4 film relies on only a CVD (chemical vapor deposition) method so that a surface level density of a silicon interface tends to increase.
In the actual photoelectric conversion by photodiode, carriers to be photoelectrically converted decrease due to a recombination of carriers on the surface.
Accordingly, when the surface level density increases as described above, a ratio at which the carriers are recombined on the surface increases so that a ratio of effective carriers to be photoelectrically converted decreases.
In particular, as the wavelength of light becomes shorter, a ratio of photoelectrical conversion in a shallow area increases so that an influence exerted due to the surface level becomes great.
Specifically, assuming that α is an absorption coefficient of light at the wavelength λ, the absorption coefficient α increases as the wavelength λ becomes short. For example, at a wavelength of 400 nm, α=7.8×104/cm is satisfied. At a wavelength of 780 nm, α=1.1×103/cm is satisfied.
Assuming that Pio is light intensity on the silicon surface, light intensity Pix in a depth x is expressed by the following equation (4):Pix=Pio·exp(−αx)  (4)
Since the absorption coefficient α increases as the wavelength λ becomes short as described above, this light intensity Pix is lowered as the wavelength λ becomes short.
Therefore, as the wavelength becomes short, the light intensity is considerably attenuated according to the depth. As a result, the ratio at which carriers are to be photoelectrically converted in the shallow area increases.
In actual practice, in the case of the antireflection film having the single layer of the Si3N4 film deposited by a low-pressure CVD method, it is observed that a photosensitivity in the short-wavelength range (laser wavelength λ<500 nm) suddenly declined.
In order to solve the above-mentioned problems, according to the present invention, there are provided an antireflection film capable of both decreasing a reflectance and lowering a surface level density on a photo receiving region of a semiconductor device to obtain a semiconductor device including a photo receiving region having high photosensitivity and a manufacturing method therefor.